The present invention relates to a method for producing a viewing angle compensator for a liquid crystal display which can diminish the viewing angle dependency of the liquid crystal display.
Liquid crystal display is occupying an important position in the display field, taking the place of the cathode-ray tube, because of such characteristic features as being driven at a low voltage, light weight and low cost. However, since the liquid crystal display is based on a method of displaying images by utilizing orienting behaviors of a liquid crystalline substance having an optical anisotropy, it inevitably involves the problem (hereinafter referred to as "viewing angle dependency") that the color tone of screen changes according to directions of seeing the panel. For example, in the case of a lap-top type personal computer or a word processor display, utilizing a twisted nematic mode ("TN" hereinafter) or a super-twisted nematic mode ("STN" hereinafter), relatively good images are obtained when the screen is seen, from the front, but when the screen is seen from a direction other than the front, the screen is colored or it becomes difficult to see images on the screen. Such viewing angle dependency of the display is not only undesirable to users but also is an obstacle to the realization of a larger screen which is required for a wall-mounted TV or the like. In the case of a large screen display, as long as the prior art is used, it is impossible to obtain clear images throughout the entire screen because the marginal portion comes to have a certain viewing angle even when the screen is seen from the front.
In connection with liquid crystal display, various display methods have been proposed. Particularly, it has been reported that a birefringence material having a negative optical anisotropy in the thickness direction exhibits a viewing angle compensating effect with respect to a display of the type wherein the state with liquid crystal molecules being oriented perpendicularly to a substrate is assumed to be a dark display. For example, it is disclosed in Japanese Patent Laid Open Nos. JP2-15237A, 2-15238A and 2-15239A that a birefringence layer having a negative optical anisotropy, obtained by a single or two sheets of oriented film, exhibits a viewing angle compensating effect for a TN display of a normally white mode or for an Electrically Controlled Birefringence (ECB) type display based on a Deformation of Aligned Phase (DAP) method. However, in the case where the compensation is to be effected with a single sheet of oriented film, the film must be completely uniaxial, but the production of such an oriented film is generally difficult, and in the case where two sheets of biaxial film are put one on the other, it is necessary that both films should be completely the same in three-dimensional refractive index and film thickness; besides, preciseness is required for the axial angle of each film. Thus, problems exist in point of manufacture and arrangement. Also, there has been proposed a method (Japanese Patent Laid Open No. JP3-67219A) wherein a film with a cholesteric structure solidified is produced using a liquid crystalline polymer such as, for example, a copolymer of benzyl-L-glutamate and dodecyl-L-glutamate and it is used as a viewing angle compensator. The use of such a liquid crystalline polymer involves problems; for example, the orienting process requires much time and labor and it is difficult to control the film thickness which is absolutely necessary for attaining a satisfactory viewing angle compensating effect.
The present iventors have already invented an optical element constituted by a nematic liquid crystalline polymer film having a twisted structure as a color compensator for TN (Japanese Patent Application No. 165718/1990) and STN (Japanese Patent Laid Open No. JP3-87720A). One important reason for the attainment of the said invention is that a liquid crystalline polymer containing ortho-substituted aromatic units, or a composition thereof, used in the invention can easily take a twisted nematic structure of monodomain. Again taking note of the good orienting property of a liquid crystalline polymer containing an ortho-substituted aromatic unit and the easiness of carrying out the orienting process, the present inventors considered it possible to produce a film having an intra-plane refractive index substantially averaged and larger than a refractive index in the thickness direction, that is, a film having a structure capable of being regarded substantially as a negative uniaxial structure has a sharper (shorter pitch) twisted structure than that required for a color compensator could be solidified. And as a result of earnest studies made on the basis of this thinking, we finally accomplished the present invention.
More specifically, in connection with the production of a compensator for a liquid crystal display, it is the object of the present invention to provide a method for producing a viewing angle compensator for a liquid crystal display using a liquid crystalline polymer film having a refractive index distribution wherein intra-plane refractive indices are approximately the same and larger than a refractive index in the thickness direction.